Support aliasing an interface method as a class member

[jonmeow]

Description of the bug:

This is the feature version of #2583 (now that the crash is fixed), to support aliasing an interface method as a class member.

What did you do, or what's a simple way to reproduce the bug?

package sample api;

interface Printable {
  fn PrintIt[self: Self]();
}

impl String as Printable {
  fn PrintIt[self: String]() {
    Print(self);
  }
}

class Vector(T:! type) {
  var x: T;

  alias PrintIt = Printable.PrintIt;
}

// Conditionally implement the API for certain `T`s.
impl forall [U:! Printable] Vector(U) as Printable {
  fn PrintIt[self: Self]() {
    Print("{{");
    self.x.PrintIt();
    Print("}");
  }
}

fn Main() -> i32 {
  var v: Vector(String) = {.x = "test"};
  v.(Printable.PrintIt)();
  v.PrintIt();
  return 42;
}

What did you expect to happen?

That the alias makes using the member name equivalent to using a qualified member name in a call. Basically, lines 30 and 31 should be equivalent:

  v.(Printable.PrintIt)();
  v.PrintIt();

What actually happened?

Currently unsupported.

Any other information, logs, or outputs that you want to share?

No response

[redbopo]

I think this is a small fix, the former will cause runtime format error.

Print("{");

->

Print("{{");

[jonmeow]

@redbopo Thanks, fixed the example. That's the result of a separate bugfix I recently put in. :)

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[chandlerc]

Closing explorer-specific issues as not-planned for now due to our decision to prioritize working on the toolchain over other implementation work in the near term: https://github.com/carbon-language/carbon-lang/blob/trunk/proposals/p3532.md